MEMS device with independent rotation in two axes of rotation

ABSTRACT

A MEMS arrangement is provided that has a top plane containing a rotatable element such as a mirror. There is a middle support frame plane, and a lower electrical substrate plane. The rotatable element is supported by a support frame formed in the middle support frame plane so as to be rotatable with respect to the frame in a first axis of rotation. The frame is mounted so as to be rotatable with respect to a second axis of rotation. Rotation in the first axis of rotation is substantially independent of rotation in the second axis of rotation.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/828,050 filed Jun. 30, 2010, which is a continuation of U.S.application Ser. No. 12/432,607 filed Apr. 29, 2009, which claims thebenefit of U.S. Provisional Application No. 61/048,724 filed Apr. 29,2008, hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to MEMS devices with rotation in two axes ofrotation.

BACKGROUND OF THE INVENTION

The first generation of MEMS (micro-electromechanical systems)wavelength selective switches used single axis tiltable mirror arrays(one mirror per wavelength) to arbitrarily switch any set of opticalwavelength signals incident at an input port to any of N output ports. Atypical configuration was to disperse the wavelengths in a first axis(y) and switch in the orthogonal axis (x). Optimizing wavelength channelshape requires a tight beam waist in the y-axis at the MEMS mirrorplane, while optimizing the number of achievable ports within a limitedMEMS tilt range leads to a large beam waist in the x-axis. It istherefore advantageous to have a mirror array with an x-axis dimensionsignificantly larger than y-axis dimension.

In order to achieve “hitless switching” (i.e. avoid scanning throughintermediate ports), a 2-axis tilt is required for each mirror. A 2Dgimbal arrangement that can fit within the footprint of the mirror canbe used to produce an arrangement with a high fill factor. The 2D gimbalcan be placed at the center of the mirror, with the disadvantage thatthe optical beam can not be centered on the mirror and does not fullyutilize the available area. A hidden gimbal approach has been reportedas a means to achieve the 2-axis tilt while maximizing the usable mirrorarea.

A significant challenge for both versions of 2D gimbals is that controlof x and y axis tilt is not independent. This is due to coupling betweendrive electrodes as a result of shared electrostatic cavities. Thiscoupling leads to a requirement for careful 2D calibration and controlin order to follow the required “hitless” path trajectory. As a result,the switch path is broken into many smaller steps, at a cost tocalibration time and to switching time.

SUMMARY OF THE INVENTION

Embodiments of the present invention substantially separate the two axesof tilt so that they can be controlled and optimized independently,while still achieving the desirable features of a high fill factor in atleast one axis. In some embodiments, de-coupling of the x and y tiltdrives allows control to be simplified and for the tilt around theelongated x-axis (i.e. roll) to be reduced to a binary 2-stateoperation. Since the mirror edge deflection when tilting around thex-axis is relatively small, and the moment of inertia is small comparedto tilting around the y-axis, the drive characteristics for roll aroundthe x-axis can be optimized to allow a relatively low drive voltage (<50V).

Tilting around the y-axis (ie. piano tilt) produces a large deflectionat the mirror tip and therefore may require larger clearances. Thelarger moment of inertia may also require stiffer hinges in order toavoid vibration and shock sensitivity. Electrostatic parallel plateactuation with high drive voltage and typically bi-directional actuationmay be employed in order to address these issues. A stepped electrodeprovides some improvement.

According to one broad aspect, the invention provides a MEMS arrangementcomprising: a top plane comprising a rotatable element; a middle supportframe plane, a lower electrical substrate plane; wherein the rotatableelement is supported by a support frame formed in the middle supportframe plane so as to be rotatable with respect to the frame in a firstaxis of rotation; wherein the frame is mounted so as to be rotatablewith respect to a second axis of rotation; wherein rotation in the firstaxis of rotation is substantially independent of rotation in the secondaxis of rotation.

According to another broad aspect, the invention provides the MEMSdevice comprising: a frame that supports a rotatable element so as toallow the rotatable element to rotate about a first axis of rotation; afirst pair of interconnections that connect the frame to a pair ofsupports so as to allow the frame to rotate about a second axis ofrotation; a first actuator for actuating rotation of the rotatableelement in the first axis of rotation, a second actuator for actuatingrotation of the frame in the second axis of rotation; the first actuatorformed so as to rotate with the frame about the second axis of rotation.

According to another broad aspect, the invention provides anelectrostatic actuator comprising: first and second vertical combsarranged to provide a comb drive, the first vertical comb connectable toa first voltage and the second vertical comb connectable to a secondvoltage; an electrostatic plate arrangement comprising a first plate anda second plate, the first plate connectable to the first voltage and thesecond plate connectable to the second voltage; wherein application ofthe first and second voltages actuates an attractive force between thecombs that brings the combs closer together and in so doing brings thefirst plate and second plate closer together such that an attractiveforce between the first plate and the second plate brings the firstplate and the second plate closer together.

According to another broad aspect, the invention provides a MEMS devicecomprising: an element to be rotated about an axis of rotation; theelectrostatic actuator summarized above wherein: one of the first andsecond plates being formed to rotate with the element to be rotated, andthe other of the first and second plates being in a static position; oneof the first and second vertical combs being formed to rotate with theelement to be rotated, and the other of the first and second verticalcombs being in a static position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a MEMS arrangement provided by anembodiment of the invention;

FIG. 1B is a plan view of layers of the MEMS arrangement of FIG. 1A;

FIG. 2 is a more detailed partial cut-away perspective view of the MEMSarrangement of FIG. 1A;

FIG. 3 is a perspective view of the base layer and bonding posts;

FIG. 4 is a perspective view of the base layer and bonding posts andstationary combs;

FIG. 5 is a perspective view of the base layer and bonding posts,stationary combs, and support frame;

FIG. 6 is a side view of MEMS arrangement with a push-pull comb drive;

FIG. 7 is a top view of a MEMS arrangement with uneven mass distributionin the mirror;

FIG. 8 is a plan view of an example upper frame layer implementation forthe embodiment of FIG. 6;

FIG. 9 is a plan view of an example lower frame layer implementation forthe embodiment of FIG. 6

FIG. 10 is a plan view of an example standoff layer implementation forthe embodiment of FIG. 6;

FIG. 11 is a plan view of an example lower support layer implementationfor the another embodiment featuring a different arrangement for they-hinges;

FIG. 12 is a plan view of an another embodiment;

FIG. 13 is a plan view of an another embodiment in which the x-axis isoffset from the center of the frame and from the center of the mirror;

FIG. 14 is a plan view of another embodiment featuring comb drive forboth x and y tilt;

FIG. 15 is a plan view of another embodiment featuring push-pull combdrive configurations for both x and y tilt;

FIG. 16 is a plan view of another embodiment in which a pair of combdrives are used for x-tilt drive, but the x-tilt combs are placedoutside the mirror area;

FIG. 17 is a plan view of another embodiment in which y-hinges arelocated outside the frame area;

FIG. 18 is a side view of a hybrid drive arrangement combining combdrive and electrostatic plate drive;

FIGS. 19 and 20 are plan views of a staggered contact arrangement;

FIG. 21 is a plan view of a MEMS arrangement featuring two rows of MEMSdevices;

FIGS. 22 to 27 are diagrams showing an example of a process forimplementing some of the embodiments;

FIG. 28A is a block diagram of a MEMS system provided by an embodimentof the invention;

FIG. 28B is a flowchart of a method of controlling a MEMS deviceprovided by an embodiment of the application;

FIGS. 29A and 29B are perspective views of a MEMS arrangement featuringa single layer support layer with electrically isolating trenches; and

FIG. 30 is a perspective view of MEMS arrangement featuring electricalbias lines running along a substrate.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A disadvantage of conventional MEMS designs featuring x and y tilt isthat high voltage drives are usually required for both axes andbi-directional tilt is often required to achieve enough tilt rangearound the y-axis, leading to three high voltage drives per mirror.Since high voltage drives require special attention due to increasedprobability of electrical breakdown/shorting, and high voltage driveelectronics are significantly more expensive than their lower voltagecounterparts, it is beneficial to minimize their use.

In addition, if a MEMS mirror forms part of a capacitive, parallel plateelectrostatic actuator, then the force exerted on the mirror, andtherefore the degree of actuation, is significantly impacted by anychanges in the flatness of the mirror. Since material stresses canchange over time and temperature, thereby affecting mirror flatness, itis advantageous to de-couple the mirror surface from the electrostaticactuation—particularly for axes that are significantly elongated (wherechanges in flatness lead to larger changes in electrostatic gaps).

Referring now to FIGS. 1A, 1B and 2 shown are perspective (FIGS. 1A and2) and layer-by-layer (FIG. 1B) of a MEMS device with two dimensionalrotation provided by an embodiment of the invention. In the particularexample illustrated, the structure is used to provide a mirror that canbe rotated independently in x and y axes of rotation; however, it is tobe clearly understood that there may be other applications of thisstructure to rotate elements other than mirrors such as a thin-filminterference filter, a reflective grating structure, or a resonantcavity etalon to name a few specific examples. FIG. 1A shows three suchdevices 80,82,84 arranged in a row from the top of the Figure to thebottom of the Figure; only the bottom device 80 of the three deviceswill be described in detail.

The MEMS device 80 is defined within a layered structure that includes atop layer 10, upper frame layer 12, lower frame layer 14 and bottomlayer 16. FIG. 1B shows a plan view of an example implementation of thetop layer 10, upper frame layer 12, lower frame layer 14, and bottomlayer 16. A legend is indicated at 70 that shows how components formingpart of each layer are illustrated. Specifically, components in the toplayer 10 are indicated with hatching 11, components in the upper framelayer 12 are indicated with hatching 13, components in the lower framelayer 14 are indicated with hatching 15, and components in the bottomlayer 16 are indicated with hatching 17.

In the top layer 10 there is defined a pair of hinges 30, 32, hingeanchors 34,36 and a mirror 38 shown partially cut away. The hinges 30,32connect opposite ends of the mirror 38 to the hinge anchors 34,36. Anaxis of rotation 39 is defined by the hinges 30,32 and will be referredto as the x-axis, and as such hinges 30,32 will be referred to as“x-hinges”. At the instant depicted in FIG. 1, the mirror 38 is shown ina slightly rotated state about the x-axis 39.

A frame generally indicated at 40 supports the components in the toplayer 10. The frame 40 is defined partially in the upper frame layer 12and partially in the lower frame layer 14. The frame 40 is supported bya pair of support blocks 42,43 (only support block 42 visible in theview of FIG. 1A; both blocks 42,43 are visible in FIG. 1B) formed in thebottom layer 10 through a pair of hinges 46,48 formed here in the upperframe layer 12 (or alternatively in the lower frame layer 14, or bothupper frame layer 12 and lower frame layer 14) that allow the frame 40to rotate about an axis of rotation 41 that will be referred to hereinas the y-axis. As such, hinges 46,48 will be referred to as “y-hinges”.A respective one of the y-hinges 46,48 connects the frame 40 to each ofthe pair of support blocks 42,43. At the instant depicted in FIG. 1, theframe 40 is shown in a slightly rotated state about the y-axis 41.

The hinge anchors 34,36 in the top layer 10 are supported by a portionof the frame 40 formed in the upper frame layer 12. The mirror 38 andoptionally the x-hinges 30,32 are thinner than the rest of thecomponents formed in the top layer 10 namely the hinge anchors 34,36such that there is a gap, referred to herein as the “x-gap”, between themirror 38 and the upper frame layer 12; the x-gap provides space for themirror 38 to rotate in the x-axis 39.

The frame 40 includes a first moveable y-tilt vertical comb 60. In theillustrated example, this is defined in the upper frame layer 12. Thefirst moveable y-tilt vertical comb 60 moves with the frame 40 when theframe 40 rotates about the y-axis 41. A support 64 defined in the bottomlayer 10 supports a second static y-tilt vertical comb 62. The secondy-tilt vertical comb 62 is defined in the same layer used for the lowerframe layer 12, but the second y-tilt vertical comb is not connected tothe frame 40 and does not move with the frame; rather, the second y-tiltvertical comb 62 is statically connected to the support 64.

Also shown is a pair of x-tilt electrodes 50,52 defined in the upperframe layer 12. The x-tilt electrodes 50,52 are offset from the x-axis39. As detailed below, the MEMS device 80 allows for an x-voltage to beapplied to the pair of electrodes 50,52.

Three traces 70,72,74 deliver voltages to the device. Specifically,trace 70 delivers an x-voltage; trace 72 delivers a y-voltage; and trace74 delivers a ground voltage. For the purpose of enabling control ofrotation in the x-axis 39, there is a conductive path connecting thetrace 70 delivering the x-voltage to the x-tilt electrodes 50,52, andthere is a conductive path connecting the mirror 38 to ground trace 74.For the purpose of controlling rotation in the y-axis 41, there is aconductive path connecting the static y-tilt vertical comb 62 to thetrace 72 delivering the y-voltage, and there is a conductive pathconnecting the movable y-tilt vertical comb 60 to the ground trace 74.These conductive paths will be described in further detail below.

In operation, the application of an x-voltage to the x-tilt electrodes50,52 and the ground voltage to the mirror 38 results in a potentialdifference between the x-tilt electrodes and the mirror 38; the x-tiltelectrodes 50,52 are offset from the x-axis 39 such that the resultingelectrostatic attraction causes the mirror to rotate about the x-axis39. When the x-voltage is returned to ground, the tension in thex-hinges 30,32 returns the mirror 38 to its default position in whichthere is no rotation about the x-axis 39.

Similarly, the application of a y-voltage to the static y-tilt verticalcomb 62 and the ground voltage to the moveable y-tilt vertical comb 60results in a potential difference between the y-tilt vertical combs60,62. The resulting electrostatic attraction between the combs 60,62causes the frame 40 to rotate about the y-axis 41. The mirror 38 issupported by the frame 40 and as such experiences the same rotationabout the y-axis. When the y-voltage is returned to ground, the tensionin the y-hinges 46,48 returns the frame 40 (and the mirror 38) to itsdefault position in which there is no rotation about the y-axis 41. Theheight of the supports 42,43 defines a limit on the amount of rotationpossible in the y-axis 41.

In the above described embodiment, the mirror 38 can roll (x-tilt) withrespect to the support frame 40 containing drive combs 60,62,electro-static x-electrodes 50,52, y-tilt hinges 46,48, and the x-tilthinges 30,32 can be formed in the same layer as the mirror at both endsof the mirror.

In some embodiments, the arrangement is implemented using a 3 layerstack. Each layer may include one or more sub-layers. The top-most layeris used for the mirror and x-hinges. The middle layer is used for thesupport frame and upper and lower part of the vertical comb drive, andthe bottom-most layer is used for the supports and paths to deliver thevoltages used to control the rotation.

In a specific example, the top-most layer is formed from SOI (silicon oninsulator) which includes a removable handle wafer, a Silicon layer andan Oxide layer; the middle frame is formed from DSOI (double silicon oninsulator) comprising a handle wafer and two Si layers and two Oxidelayers. Doped silicon can be used for SOI and DSOI to provide electricalcontinuity without the need for metal traces. The oxide in DSOI providesan etch stop for Deep Reactive Ion Etching (DRIE) and electricalisolation between upper and lower halves of middle frame. Conductivevias can be used to provide electrical connection between frame layerswhere desired. The upper and lower frame layers can therefore operatetogether as a combined mechanical structure while allowing isolatedelectrical routing to desired locations. Detailed examples are givenbelow.

With the design illustrated, the x-gap can be small since the mirrordeflection is small during x-tilt. Advantageously, this allows for arelatively low voltage x-tilt drive compared to the voltage used todrive rotation in the y axis.

It can be seen that x and y tilt are substantially decoupled.

Rotation in the x-axis is achieved as a result of an attractive forcebetween the x-electrodes formed in the frame layer, and the mirror layerwhich is connected to ground. This rotation in the x-axis is noteffected by the state of y-rotation. This is because the distancebetween the mirror and the x-electrodes is not a function of the stateof y-rotation due to the fact that the x-electrodes rotate with theframe along with the mirror when there is rotation in the y-axis.

Rotation in the y-axis is achieved as a result of an attractive forcebetween the static drive comb in the lower frame layer and the movablecomb in the upper frame layer. This rotation in the y-axis is noteffected by the state of x-rotation. This is because the distancebetween the drive combs is not a function of the state of x-rotation.X-rotation only rotates the mirror; there is no rotation of the frame inthe x-axis, and in turn, there is no rotation of the movable comb in thex-axis.

Wavelength selective switch applications are typically less sensitive tox-axis rotation, and as such the resulting device can be quiteinsensitive to vibration in x-roll. This means that the x-tilt hingescan be weaker than they might otherwise need to be and this in turn alsolowers the voltage needed to actuate rotation in the x-axis.

The resulting wavelength selective switch device can also be quiteinsensitive to mirror flatness as described previously since changes inthe mirror to frame separation along the long axis 39 do not impacty-tilt. Furthermore, flatness along the short axis 41, which wouldimpact x-tilt, is less critical.

In some embodiments, the drive for x-tilt is implemented with twostates, namely a binary ON state during which a predetermined x-voltageis applied, and binary OFF during which an x-voltage of zero (or anypredetermined secondary voltage) is applied.

The following is a detailed set of example dimensions for the variouslayers. The actual dimensions would be implementation specific.

Top SOI Layer: The silicon layer of the upper most layer can for examplebe 10-20 microns thick, and is used to form the grounded mirror andx-tilt hinges. A gold reflector (or other highly reflective layer) isused to implement the mirror functionality. A backside etch is performedto form a 5-10 micron x-gap between the mirror and the top frame layer.The x-hinges may also be thinner than the hinge anchors.

DSOI layer—upper layer: the silicon top layer can for example be 10-15microns thick and is used to form a grounded portion of the frame (andis connected to the mirror through x-tilt hinges), x-tilt electrodeislands (Vx), moveable y-tilt vertical combs (connected to groundedportion of the frame), y-tilt hinges (one connected to ground, the otherconnected to x-voltage).

DSOI layer—lower layer: the silicon layer (approx 10-15 microns) is usedto form the static y-tilt vertical combs, bridge sections to anchor thex-tilt electrode islands to the ground portion of the frame formed inthe upper layer of the DSOI layer; electrical vias connect the firstgrounded y-hinge to upper ground frame and the second y-hinge conductingVx to the x-tilt electrodes.

Bottom via substrate is used for through wafer vias, contact metal forVy, Vx and Gnd, etched stand-offs (supports) to create clearance fory-tilt.

In some embodiments, the combination of stiff comb drive tilt, etchedframe, and large stand-off clearance should effectively eliminatepneumatic effect which can cause transient coupling between adjacentmirrors.

A detailed view of the electrical traces 70,72,74 and the supports42,43,64 is shown in FIG. 3.

A detailed view of static y-tilt vertical comb 62 is shown in FIG. 4;this figure also shows sections 100,118 of the lower frame layerreferred to below in the description of the delivery of the variousvoltages used to control the rotation.

A detailed view of the upper frame layer is shown in FIG. 5.

With reference to FIG. 2, the x-voltage is delivered to the x-tiltelectrode 52 as follows:

1) x-voltage applied to trace 70;

2) conducts up support 42;

3) conducts up through isolated section 100 of lower frame layer;

4) conducts to section 102 of upper frame layer through via 108 thatconnects isolated section 100 of lower frame layer to section 102 ofupper frame layer that would otherwise be electrically insulated due tooxide layer in between;

5) conducts through y-hinge 46;

6) conducts to section 104 of upper frame layer;

7) conducts to section 112 of lower frame layer through via 118 thatconnects section 104 of upper frame layer to section 112 of lower framelayer that would otherwise be electrically insulated;

8) conducts to x-electrode 52 through via 114 that connects the x-tiltelectrode 52 to section 112 of lower frame layer.

The x-voltage is delivered to x-tilt electrode 50 as follows:

1) conducts to section 112 of lower frame layer as detailed above in thedescription of the delivery of the x-voltage to x-tilt electrode 52;

2) conducts around entire lower frame (not shown in FIG. 2 but can beseen in FIG. 1B) until it reaches section 114 of lower frame layer;

3) conducts to x-tilt electrode 50 through via 116 that connects thex-tilt electrode 50 to section 115 of the lower frame layer.

The ground voltage is delivered to the mirror 38 as follows:

1) ground applied to trace 74;

2) conduct up other support 43;

3) conduct to isolated section 118 of lower frame layer;

4) conduct to section 120 of upper frame layer through via 122 thatconnects isolated section 118 of the lower frame layer to section 120 ofthe upper frame layer;

5) conduct through y-hinge 48;

6) conduct through section 120 of upper frame; section 120 of framesupports hinge anchor 36;

7) conduct through hinge anchor 36;

8) conduct through x-hinge 32 to mirror 38.

Note the ground voltage also reaches the mirror through the otherx-hinge 30 in a similar manner.

The y-voltage is delivered to the static y-tilt vertical comb 62 asfollows:

1) y-voltage applied to trace 74; trace 74 is connected to support 64;

2) conducts through support 64 to static y-tilt vertical comb 62.

The ground voltage is delivered to the moveable y-tilt vertical comb 60as follows:

1) ground voltage conducts to section 120 of upper frame layer asdescribed above when describing the delivery of ground voltage to mirror38; moveable y-tilt vertical comb 38 is connected to section 120 of theupper frame layer;

2) conduct to movable y-tilt vertical comb 38.

Referring now to FIG. 6, shown are side views of a variant of theabove-described MEMS device. Generally indicated at 310 is a side viewin which the y-tilt is not actuated. Generally indicated at 312 is aside view in which the y-tilt is actuated. Generally indicated at 330 isan example of a layer stack that might be used to implement thisembodiment. The layers include a silicon substrate 344; a 1 micron oxidelayer 342; a 40 micron silicon stand-off layer with trace regionsthinned to 10 microns 340; a 10 micron lower silicon frame layer 338; a1 micron oxide layer with conductive vias in selected regions 336; a 15micron silicon frame layer 334; upper silicon frame layer 332 and a 15micron silicon top layer with mirror regions thinned to 11.5 and 10 μm.The thinning of the mirror to 11.5 and 10 microns is detailed furtherbelow with reference FIG. 7. It is of course to be understood that thesedimensions and materials are for the purpose of example only. With thisembodiment, there are two sets of y-tilt vertical combs; a first setincludes a moveable comb 314 in the upper frame layer and a static comb316 in the lower frame layer, as in the above-described embodiment, andthe second set includes a moveable comb 318 in the lower frame layer anda static comb 320 in the upper frame layer. Note that for the embodimentof FIG. 6, a push-pull configuration for the y-tilt is provided in whichactuation is achieved through a simultaneous pull up on one side andpull down on the other side. This is in contrast to the embodiments ofFIGS. 1 through 5 where the y-tilt electrodes only pull in onedirection.

FIG. 7 shows a plan view of the top silicon layer for the embodiment ofFIG. 6. The top layer includes the mirror 34 which has areas of twothicknesses. A first area 342 is thinned to 11.5 μm while the secondarea 344 is thinned to 10 μm. The x-hinge thickness is set to give afirst resonance frequency at around 1 kHz. The x-hinges in this exampleare approximately 15 μm thick by 1.5 μm wide, and have approximately 500to 1000 μm effective length. Also shown are two hard stops 346 which arethinned to 11.5 microns. The hardstops 346 provide a small contactsurface which limits the range of x-tilt motion (particularly useful forthe binary control mode)—the small contact area limits stiction whichcould prevent the mirror from returning to a rest position. Also, theplacement of the hardstops is chosen to ensure contact is only betweenthe grounded mirror and the grounded section of the upper frame, whilealso creating a minimum separation, for example of 1.5 microns, betweenthe grounded mirror and the x-electrode at full x-tilt.

FIG. 8 shows a plan view of the upper frame layer for the embodiment ofFIG. 6. In this example, the upper frame layer is 15 microns thick. Theanchor posts 350 are 28×28 μm and have 3 μm lateral clearance. The xelectrodes 352 are 35×450 μm. The y-hinge 355,356 stiffness is set togive a first resonance frequency of around 1 kHz. They are about 15 μmthick by 1.5 μm wide and have approximately 100 to 200 μm effectivelength. The comb fingers 358 are 3 μm wide, have a 12 μm pitch and areapproximately 50 to 100 μm long. In another example, the comb fingersare 75 to 150 μm long, for example 100 μm long. The frame has breaks 360for isolation. The side of the frame is approximately 5 μm as indicatedat 362. The overall length of the frame 364 is about 1.4 mm. The end ofthe frame 366 is about 30 μm. The x electrodes 352 are each placed about25 μm from the ends of the frames. The width 370 of the frame is about76 μm.

FIG. 9 shows a plan view of the lower frame layer for the embodiment ofFIG. 6. The lower frame layer has 5 μm wide frame sections 380. Thereare conductive vias 382 that connect the upper and lower frame layers.There are breaks in the frame for isolation as illustrated. There arecomb fingers 384 that are 3 μm wide, have a 12 μm pitch are 50 to 100 μmlong. In another example, the comb fingers are 75 to 150 μm long, forexample 100 μm long. These overlap the comb fingers 358 of FIG. 8 byapproximately 40 to 90 microns. In another example, these overlap thecomb fingers 358 of FIG. 8 by approximately 65 to 140 microns. The widthof the lower frame layer 386 is about 76 μm. The length 388 of the lowerframe layer is about 1.4 mm.

FIG. 10 shows the support/stand-off layer for the embodiment of FIG. 6.Shown are stand-offs 390 for the y voltage, stand-off 392 for the xvoltage and stand-off 394 for ground voltage, all for delivering thesevoltages to the frame layers.

FIG. 11 shows plan views of a lower support layer generally indicated at140 and an upper support layer generally indicated at 142 for anotherdesign variant. Note that these layers are shown for two differentadjacent devices. The lower support layer 140 corresponding with theupper support layer 142 would be a reflection in the x-axis of thatshown at 140. For the example of FIG. 11, the electrode layout issimilar to the embodiment of FIGS. 1 through 5. The arrangement of they-hinges 400,402 is somewhat different with the hinges shown connectedto the upper frame layer and extending generally in the direction of they-axis to connect to the supports. However, the function of these hingesis generally the same as that of hinges 46,48 of FIG. 1, namely to allowrotation of the frame in the y-axis.

Referring to FIG. 12, shown is another embodiment that is similar tothat of FIG. 11, but which features a post and y-hinge arrangementsimilar to the embodiment of FIGS. 1 through 5.

Referring now to FIG. 13, shown is another variant in the design inwhich the x-axis 418 is offset from the center of the frame and from thecenter of the mirror. This is achieved by having the x-hinges 420,422offset from the central position. Advantageously, this results in alarger area of the mirror being available for the purpose of x-drivetorque. In some embodiments, mass balancing of the mirror is performedby including frame material in portions of the mirror on the side of thex-axis where the mirror is smaller. In a specific example of this, massin the frame layer in area 424 is included beneath the mirror to makethe portion of the mirror on that side of the axis approximately equalin mass to the portion on the other side of the axis. In a specificexample implementation for the embodiment of FIG. 13, the mirror isapproximately 80 microns wide by 1200 microns in length and has portionsthat are 10 microns thick and 46 microns thick where the frame layersare attached. The x-hinges are attached so that the x-axis of rotationis located 54 microns from one mirror edge and 26 microns from the otheredge. The volume of silicon material on Side 1 (W×L×T)=54×1200×10microns=648 k μm^3. The distance from mass centroid of side1 tox-axis=27 microns. The distance from mass centroid of side2 to x-axis=13microns. Therefore, side2 needs to have 2× larger mass than side 1. Side2 (W×L×T)=26×850×46 microns+26×350×10 microns=1108 k μm^3 (close tobalanced). This example results in balancing the torque from the 2sides; since their mass centroids are not equal distances from the tiltaxis, the masses have been to be adjusted to compensate.

Referring now to FIG. 14, shown is another variant in which a comb driveis provided for both x and y tilt. In the illustrated example, there isa pair of vertical combs 450,452 for y-drive and two pairs of verticalcombs 454,456 and 458,460 for x-drive. The embodiment of FIG. 14 alsofeatures an offset x-axis and mass balancing as described above for theembodiment of FIG. 13 but it is to be understood that the comb drive forthe x-axis could also be implemented in an embodiment without an offsetx-axis. The pairs of vertical combs for x-drive include a rotor 450 inthe upper frame layer connected to the mirror and a stator 452 in thelower frame that is connected to the frame. The y combs are as describedpreviously with reference to the embodiment of FIG. 1A.

FIG. 15 shows another variant featuring push-pull comb driveconfigurations for both the x and y tilt. The y-drive is as describedpreviously with reference to FIG. 6. the x-drive can best be understoodwith reference to the end view generally indicated at 470. There is astator comb 472 in the lower frame layer 70; a rotor comb 474 in theupper frame layer that is attached to the mirror 476; there is anotherrotor comb 478 in the lower frame layer connected to the mirrors andanother stator comb 480 in the upper frame layer. This arrangement isrepeated in either side of the y-axis.

FIG. 16 shows another variant in which pairs of vertical combs are usedfor x-tilt drive. In this embodiment, the x-tilt combs 500,502 areplaced outside the mirror area, but inside the frame (not shown). Thisapproach allows for more off-axis torque, but requires a longer framethan in the other embodiments.

FIG. 17 shows another variant which is similar to the embodiment of FIG.1A except for the fact that there are y-hinges 510,512 located outsidethe frame area. In the particular example depicted, each y-hingestraddles an area that is partially beneath the mirror and partiallybeneath the mirror of an adjacent neighbour. There are no completely“hidden” hinges for the embodiment of FIG. 17. In order to have thedevices as close together as possible, in some embodiments adjacentdevices in FIG. 17 are offset slightly from each other along the x-axisas illustrated.

Referring now to FIG. 18, shown is yet another option for electrostaticactuation. This will be referred to herein as a “hybrid” drive becauseit combines both a comb drive and electrostatic plates. Specifically,there is shown a pair of vertical combs 150,152, one of which isconnected to ground and the other of which is connected to an actuatingvoltage. In addition, the mirror (or other device to be rotated) isindicated at 154 and is connected to ground, and there is anelectrostatic plate 156 that is connected to receive the actuatingvoltage. At rest, the configuration takes the appearance generallyindicated at 160. The initial gap between the mirror 154 and theelectrostatic plate 156 is too large for actuation (at least using anyreasonable voltage) and the comb drive 150,152 dominates. The end ofphase one of actuation is indicated at 162. Assuming that the comb drivehas been actuated, that causes rotation in the rotation about the axisof rotation 166 but at a certain point, the comb drive will run out oftorque. However, the rotation thus achieved will reduce the gap betweenthe electrostatic plate 156 and the mirror 154 and create a secondarydrive. At this point, the electrostatic plate 156 and mirror 154 willdominate the drive. This allows the mirror to be rotated slightly moreto the position indicated at 164. Note that at this point, the combdrive 150,152 is overdriven, and this will provide pull back (in theopposite direction of rotation above the axis of rotation 166) to resistsnap between electrostatic plate 156 and mirror 154. This type of drivecan be employed for any of the embodiments described above featuringcomb drives. More generally, this type of hybrid drive can be applied indevices and systems other than the particular MEMS device withindependent rotation and two axes of rotation described herein. Forexample, it could be applied in a system in which rotation in a singleaxis of rotation is of interest.

Referring to FIG. 19, in some embodiments, staggered contactarrangements are employed to allow the devices to be implemented closertogether. Shown is a set of six devices generally indicated at530,532,534,536,538 and 544. In FIG. 19, the ground connections are allrouted to a common bus on the left hand side (one contact pad 550).Respective X-axis drive voltages are routed to staggered contacts 552,554,556,558,560,562 on the left side of the y-axis. Respective Y-axisdrive voltages are routed to the staggered contacts570,572,574,576,578,580 on the right side of the y-axis. This staggeredconfiguration is particularly beneficial when used with through-wafervias to the backside of the bottom substrate. With the example of FIG.19, the spacing between the contacts in the x-axis is a minimum of 250micrometers. The spacing between consecutive aligned contacts in they-axis is also 250 micrometers. The result is that any two contacts areat least 250 micrometers apart, this is not withstanding the fact thatin the y direction, the devices are spaced much closer than that, and inthe illustrated example, there are three devices within the 250micrometer spacing. Another view of this is shown in FIG. 20. Theseelectrode arrangements are advantageous when combined with through-wafervias in the lower substrate wafer so as to provide a 2-dimensional gridarray of electrical contacts on the backside of the substrate.

FIG. 21 shows another variant in which there are two rows 560,562 ofMEMS devices. This embodiment features the use of thru-wafer vias in theelectrical substrate to allow two rows of MEMS devices with closespacing between the rows. This reduces the amount of routing required.

FIGS. 22 through 27 show an example of a process for implementing theabove embodiments. This is a specific example and of course otherprocesses could alternatively be used.

It can be seen that in general, embodiments of the invention feature aframe that supports a rotatable element, such as a mirror, so as toallow the rotatable element to rotate about a first axis of rotation,such as the x-axis. In the examples described above, a pair of hinges isused to support the rotatable element to the frame in this manner. Thereis a pair of interconnections that connect the frame to a pair ofsupports so as to allow the frame to rotate about a second axis ofrotation such as the y-axis. In the illustrated embodiments, these haveincluded hinges such as shown in FIG. 1, but more generally other formsof flexible ligature can also be employed. Typically the rotatableelement is allowed to rotate about the first axis of rotation throughthe inclusion of hinges that interconnect the rotatable element to theframe. Other types of interconnections are contemplated. For example,the rotatable element could be structured as a cantilever with therotatable element bonded on one side of the entire length of the frameand thinned near the bonded edge to allow bending for x-tilt. Anothersimilar variant includes the use of compliant springs on the “free” edgeof the cantilever.

The device features a first electrostatic actuator formed so as torotate with the frame in the second axis of rotation for actuatingrotation in the first axis of rotation. This typically involves formingthe first electrostatic actuator at least partially in one or both ofthe layers used to form the frame and partially on the element to berotated; both the frame and the element to be rotated rotate with theframe in the second axis of rotation. The actuator can be internal tothe frame as in the embodiments described, or may be completely orpartially external to the frame in other implementations. While all theembodiments assume actuation by electrostatic actuator, other types ofactuators may alternatively be employed, such as electromagnetic to namea specific example. Referring to the example of FIG. 1, the x-electrodes50,52 are formed in the frame. Because of this, the x-electrodes 50,52rotate with the frame thereby allowing a very small x-gap to bemaintained. Other examples have also been described. For example, in theembodiment of FIG. 14 featuring comb drive for both x and y, each pairof combs for x-drive includes one formed in the frame and one connectedto the underside of the mirror. Once again, the proximity of the mirrorto the frame allows for a small gap between the two electrodes to bemaintained. Also important in this case, is the fact that both sets ofx-combs rotate together during y-axis tilt, thereby preventing anytwisting of the combs which could cause them to bind. In the embodimentsdescribed, there is an x-voltage applied to the x-electrodes, and groundis applied to the other electrode for actuating rotation in the firstaxis of rotation. More generally, at least for electrostatic actuatorimplementations, first and second voltages are applied, and it is thedifference between the voltages that sets up the electrostaticattraction.

A second electrostatic actuator is provided for actuating rotation inthe second axis of rotation. In the embodiments described, verticalcombs are used to provide for rotation in the y-axis. Various exampleshave been described; in FIG. 1, a single comb drive is provided for yrotation; in FIG. 6, a push-pull arrangement for the y-tilt combs isemployed in which two pairs of combs are used for y rotation. In theembodiments described, there is y-voltage applied to one of the y-tiltcombs, and ground is applied to the other y-tilt comb for actuatingrotation in the second axis of rotation. More generally, at least forelectrostatic actuator implementations, first and second voltages areapplied, and it is the difference between the voltages that sets up theelectrostatic attraction. While all the embodiments assume actuation byelectrostatic actuator, other types of actuators may alternatively beemployed, such as electromagnetic to name a specific example.

In some embodiments, the first pair of interconnections, for example they-hinges, are entirely hidden by the top mirror; in other embodiments,the first pair of interconnections are slightly exposed, for example asshown in FIG. 17 where the y-hinge is straddled in area beneath themirror and an adjacent neighbour.

In some embodiments, various techniques may be used to bond the variouslayers together. Specific examples include thermal, solder, polymer,thermo-compression, ultrasonic thermo-compression, and anodic bonding.Bond interfaces might for example include be Si/Si, Si/Silicon-Oxide,Silicon-oxide/Silicon-oxide, Silicon/metal/silicon, silicon/metal/oxide,Oxide/metal/oxide.

With reference to FIG. 28A, another embodiment provides a MEMsarrangement that has a controller 300 and a MEMs device 302. The MEMsdevice 302 is a MEMs device that is controllable to have x-tilt andy-tilt, in which x-tilt and y-tilt control are substantially decoupled.A specific example is the MEMs device described with reference toFIG. 1. The controller is implemented at least partially in hardwaresuch as a chip or controller device. The implementation may includesoftware running on the hardware. The controller 300 is connected to theMEMs device 302 through three connections 304 for delivering x-voltage,y-voltage and ground.

The operation of the arrangement of FIG. 28A will now be described withreference to FIG. 28B. The method begins in step 28B-1 with theapplication of a first predetermined x-voltage for driving x-tilt. Thiscauses rotation in the MEMs device in the x-axis away from a defaultstate. The method continues in step 28B-2 with the application of aselectable voltage to the y-voltage for driving the y-tilt. The degreeof rotation in the y-axis is controlled through appropriate selection ofthe selectable voltage. The method continues in step 28B-3 with theapplication of ground to the x-voltage. This causes rotation of the MEMsdevice in the x-axis back to the default state. It can be seen thatoperation in the x-axis is binary in nature. In some embodiments, theMEMs device is a device with high fill factor, for example >90% Theembodiments described above have featured x-hinges that are not hiddenby the upper mirror layer. In addition, the frame layer was composed oftwo layers that structurally behave as one layer but have electricallyisolated regions. Another embodiment of the invention is shown in FIGS.29A and 29B. In this embodiment, the x-hinges are hidden by the uppermirror layer, and the frame layer is formed of a single structural layerwith electrical isolation trenches to create electrically isolatedregions. It is noted that these two changes, namely the hidden x-hingesand the single frame layer with electrical isolation trenches are eachimplementable separately and can be included with any of the embodimentsas described previously. The view of FIG. 29A shows the top mirror layer600 in place whereas the view of FIG. 29B shows the top mirror layerremoved to enable depictions of the details of the frame layer with fulldetail. The frame layer 604 has a first area 620 upon which x-axisparallel plate electrostatic drive electrode 606 are located. There isalso a second section 622 of the frame 604 which is connected to ground.A pair of y-hinges 614 are shown which connect the frame 604 to y-axisanchor 610. The y-hinges 614 also deliver the ground and x-voltage tothe frame as in previous detailed embodiments. Y-drive is performedthrough a pair of y-axis vertical comb drives 616,617. The mirror 600 isconnected to the frame through x-hinges 618,619. X-axis anchor 602 shownin FIG. 29A are used to connect the top mirror layer 600 to the x-hinges618,619. Also shown are electrical control lines 624 and a lowerelectrode substrate 626. In operation, the top mirror layer 600 canrotate about the x-axis through hinges 618,619 when a voltage is appliedto the x-axis electrode 606. Rotation in the y direction is achieved byapplying a voltage across the y comb drives.

In the original MEMS design concept, a middle frame structure, comprisedof two conductive layers separated by an insulating dielectric layer,was tilted in one axis (y-tilt) relative to a lower electrode substrate;and an upper mirror layer was tilted in a second, independent axis(x-tilt) relative to the frame.

For the previously detailed embodiments, the electrodes used to generatex-tilt travelled with the frame as it tilted in the y-axis—therebyde-coupling the x and y drive calibration so that each could be treatedas independent. This continues to be the case for the embodiment ofFIGS. 29A and 29B, with the exception that the frame structure is nowcomprised of a single layer and the isolation required to provide biasedx-electrode regions is now achieved using isolation trenches. Thesetrenches are cut all the way through the frame layer during anintermediate processing step (while the frame layer is still supportedby a handle wafer, which will be subsequently removed) and thenback-filled with an insulating dielectric to maintain mechanicalintegrity while proving the necessary electrical isolation.

In the previously detailed embodiments, the serpentine hinges whichprovide compliance for x and y tilt, where formed in the upper mirrorlayer and the middle frame layer, respectively. This has the advantagethat the thickness of the x and y hinges can be chosen independently(since they are formed in separate layers during different processingsteps) but necessitates tight process control for 2 fabrication steps(since the x and y hinges typically require the tightest dimensionalcontrol). The x-hinges also reduce the usable mirror surface area.

In the design of FIGS. 29A and 29B, the x and y hinges are both formedin the frame layer, during the same processing step. Both x and y hingesare also hidden under the mirror layer so that the usable mirror surfacearea is maximized.

In any of the designs described herein, the vertical comb drives, whichare depicted for y-axis tilt, could be replaced by parallel-plateelectrostatic drives. Similarly, the parallel-plate electrostatic drivedepicted for x-axis tilt, could be replaced by a vertical comb drive.The x and y drives could therefore be any combination of parallel-plateelectrostatic, vertical comb drive, or any other suitable drivemechanism such as electromagnetic, thermal bimorph, etc.

In any of the designs described herein, electrically grounded shieldsmay be added to reduce effects of unwanted stray electromagnetic fields.For example, a grounded shield may be formed over the electrical controllines that route bias voltages to the x and y drives—thereby eliminatingany electrostatic interaction between the routing lines and thesuspended frame or mirror structures. This is illustrated in FIG. 30. Inthis example, the electrical bias lines run along the substrate as inprevious embodiments, as indicated generally at 700. A stationary groundshield 702 is formed between the substrate upon which the bias lines areimplemented and the frame. This shields signals propagating along theelectrical bias lines 700 from effecting the operation of the x- and/ory-actuators. The stationary ground shields can be formed in the samelayer used for the stand-offs. The example of FIG. 30 includes two ycomb drives 704,706 that actuate rotation around y-hinges 708,709 andelectrostatic plate actuation about the x-axis defined by the x-hinges710,712. However, it is to be clearly understood that the inclusion of astationary ground shield can be included with any of the embodimentsdescribed herein.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

We claim:
 1. A MEMS device comprising: a frame that supports a rotatableelement so as to allow the rotatable element to rotate about a firstaxis of rotation; a first pair of interconnections that connect theframe to a pair of supports so as to allow the frame to rotate about asecond axis of rotation; a first actuator for actuating rotation of therotatable element in the first axis of rotation; and, a second actuatorfor actuating rotation of the frame in the second axis of rotation; thefirst actuator formed so as to rotate with the frame about the secondaxis of rotation; the frame having insulating regions and at least twoconductive regions that are electrically isolated from each other by theinsulating regions thereby defining at least two conductive channels. 2.The MEMS device of claim 1 wherein the first actuator is a firstelectrostatic actuator and wherein the second actuator is a secondelectrostatic actuator.
 3. The MEMS device of claim 2 wherein the firstelectrostatic actuator comprises: at least one electrode formed in theframe connectable to a first control voltage; wherein the mirror isconnectable to a ground voltage such that application of the firstcontrol voltage to the at least one electrode creates a potentialdifference between the at least one electrode and the mirror therebycausing rotation in the first axis of rotation.
 4. The MEMS device ofclaim 3 further comprising: a conductive path for delivering the groundvoltage to the rotatable element that passes through one of the supportsand one of the first pair of interconnections; a conductive path fordelivering the first control voltage to the first electrostatic actuatorthat passes through the other one of the supports and the other one ofthe first pair of interconnections.
 5. The MEMS device of claim 2wherein the second electrostatic actuator comprises: a first electrodeformed in the frame so as to rotate with the frame about the second axisof rotation; a second electrode spaced from the first electrode in aposition such upon application of a first voltage to one of theelectrodes and second to the other of the electrodes creates a potentialdifference across the first electrode and the second electrode thatcauses rotation in the second axis of rotation.
 6. The MEMS device ofclaim 5 wherein the first electrode comprises a moveable comb and thesecond electrode comprises a static comb.
 7. The MEMS device of claim 6further comprising: a support that supports the static comb; aconductive path for delivering the second voltage to the moveable combthat passes through one of the supports and one of the first pair ofinterconnections; a conductive path for delivering the first voltagethat passes through the support to the static comb.
 8. The MEMS deviceof claim 2 wherein the pair of interconnections that connect the frameto a pair of supports so as to allow the frame to rotate about a secondaxis of rotation comprise a pair of hinges.
 9. The MEMS device of claim8 wherein the pair of hinges is formed so as to be only partially hiddenby the rotatable element.
 10. The MEMS device of claim 9 wherein thefirst electrostatic actuator comprises at least one plate electrodeformed in the frame, such that an actuating voltage applied to the atleast one electrode and a ground voltage applied to the rotatableelement results in rotation of the rotatable element about the firstaxis of rotation.
 11. The MEMS device of claim 2 wherein the firstelectrostatic actuator comprises a first vertical comb electrode, theMEMS device comprising a second vertical comb electrode formed on therotatable element, such that a first voltage applied to the firstvertical comb electrode and a second voltage applied to the secondvertical comb electrode results in rotation of the rotatable elementabout the first axis of rotation.
 12. The MEMS device of claim 2 whereinthe first electrostatic actuator comprises a first vertical combelectrode, the MEMS device comprising a second vertical comb electrodeformed away from but connected to the rotatable element so as to rotatewith the rotatable element, such that a first voltage applied to thefirst vertical comb electrode and a second voltage applied to the secondvertical comb electrode results in rotation of the rotatable elementabout the first axis of rotation.
 13. The MEMS device of claim 2 whereinthe second electrostatic actuator for actuating rotation in the secondaxis of rotation comprises: a pair of vertical combs.
 14. The MEMSdevice of claim 2 wherein the second electrostatic actuator foractuating rotation in the second axis of rotation comprises: two pairsof vertical combs in a push-pull arrangement.
 15. The MEMS device ofclaim 2 wherein the second electrostatic actuator for actuating rotationin the second axis of rotation comprises a pair of vertical combs and anelectrostatic plate arranged to provide a hybrid drive in which thevertical combs are engaged first, followed by engagement of theelectrostatic plate.
 16. The MEMS device of claim 2 further comprising:a plurality of conductive paths for delivery of control voltages; ashielding element that performs some shielding between the paths fordelivery of control voltages and both the support frame and therotatable element.
 17. The MEMS device of claim 1 wherein: the frame isformed in part from a first conductive layer and in part from a secondconductive layer that is insulated from the first conductive layer by aninsulating layer comprising said insulating regions except for one ormore vias that interconnect the conductive layers in specific locationsthrough the insulating layer; the at least two conductive regionscomprising the first conductive layer and the second conductive layer.18. The MEMS device of claim 1 wherein: the frame is formed from aconductive layer comprising said conductive regions with trenches filledwith insulating material comprising said insulating regions.
 19. TheMEMS device of claim 1 wherein the first pair of interconnectionscomprises a pair of hinges.
 20. The MEMS device of claim 1 wherein therotatable element is a mirror.
 21. The MEMS device of claim 1 whereinthe rotatable element is rotatable about the first axis with a pairhinges formed in a same layer as the rotatable element.
 22. The MEMSdevice of claim 1 wherein the rotatable element is rotatable about thefirst axis with a pair of hinges formed in a same layer as the frame.23. The MEMS device of claim 22 wherein the pair of hinges is entirelyhidden by the rotatable element.
 24. A MEMS device comprising: a framethat supports a rotatable element so as to allow the rotatable elementto rotate about a first axis of rotation; a first pair ofinterconnections that connect the frame to a pair of supports so as toallow the frame to rotate about a second axis of rotation; a firstactuator for actuating rotation of the rotatable element in the firstaxis of rotation; and, a second actuator for actuating rotation of theframe in the second axis of rotation; wherein the first actuator isformed so as to rotate with the frame about the second axis of rotation;wherein the first actuator is a first electrostatic actuator and whereinthe second actuator is a second electrostatic actuator; wherein the pairof interconnections that connect the frame to a pair of supports so asto allow the frame to rotate about a second axis of rotation comprise apair of hinges; and wherein the pair of hinges is formed so as to beentirely hidden by the rotatable element.
 25. A MEMS device comprising:a frame that supports a rotatable element so as to allow the rotatableelement to rotate about a first axis of rotation; a first pair ofinterconnections that connect the frame to a pair of supports so as toallow the frame to rotate about a second axis of rotation; a firstactuator for actuating rotation of the rotatable element in the firstaxis of rotation, and, a second actuator for actuating rotation of theframe in the second axis of rotation; wherein the first actuator isformed so as to rotate with the frame about the second axis of rotation;and wherein the first axis of rotation is offset from a centre line ofthe rotatable element.
 26. The MEMS device of claim 25 furthercomprising counter-balancing material in the rotatable element to offseta mass imbalance caused by the offset position of the first axis ofrotation.